Hello JC
Source document fields is invisible when there is no value store on it.
When PO is generated from SO or reordering rules then related reference for SO or reordering rules will be shown to PO In source document field.

When product route set as MTO + BUY and once we confirm SO related that product then SO reference will shown in PO.
Same goes for reordering rules if we have set reordering rule for product and when scheduler runs then related PO will show reference for Reordering rules in source document field.

Hope this will help
Thanks